Direct observation of field-induced incommensurate fluctuations in a one-dimensional S=1/2 antiferromagnet

Neutron scattering from copper benzoate, Cu(C6D5COO)(2) . 3D(2)O, provides the first direct experimental evidence for field-dependent incommensurate low energy modes in a one-dimensional spin S = 1/2 antiferromagnet. Soft modes occur for wave vectors (q) over tilde = pi +/- delta (q) over tilde(H), where delta (q) over tilde(H) approximate to 2 pi M(H)/g mu(B) as predicted by Bethe ansatz and spinon descriptions of the S = 1/2 chain. Unexpected was a field-induced energy gap Delta(H) alpha H-alpha, where alpha = 0.65(3) as determined from specific heat measurements. At H = 7 T (g mu(B)H/J = 0.52), the magnitude of the gap varies from 0.06J to 0.3J depending on the orientation of the applied field.